snippetjavascriptTip

How can I deep freeze an object in JavaScript?

Submitted by: @import:30-seconds-of-code·Mar 2, 2026·

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Problem

Objects in JavaScript are mutable, regardless if you define them as const variables or not. In fact, using const when defining an object only prevents the variable from being reassigned. However, you can reassign the properties of a const object or array.
In order to make an object immutable, you can use either Object.freeze() and Object.seal(). Although similar, they have a key difference that you need to remember.
Both of these methods prevent new properties from being added and existing properties from being removed. Additionally, Object.freeze() also prevents existing properties from being altered. The reason for that is that Object.seal() only marks existing properties as non-configurable, meaning their values can be changed as long as they are writable.
| | Create | Read | Update | Delete |
| --- | --- | --- | --- | --- |

Solution

const myObj = { a: 10, b: 20, c: 30 };
myObj.a = 12;     // { a: 12, b: 20, c: 30 };

const myArr = [15, 25, 35];
myArr[1] = 28;    // [15, 28, 35];

Both of these methods prevent new properties from being added and existing properties from being removed. Additionally, Object.freeze() also prevents existing properties from being altered. The reason for that is that Object.seal() only marks existing properties as non-configurable, meaning their values can be changed as long as they are writable.
| | Create | Read | Update | Delete |
| --- | --- | --- | --- | --- |
| Object.freeze() | No | Yes | No | No |
| Object.seal() | No | Yes | Yes | No |
> [!TIP]

Code Snippets

const myObj = { a: 10, b: 20, c: 30 };
myObj.a = 12;     // { a: 12, b: 20, c: 30 };

const myArr = [15, 25, 35];
myArr[1] = 28;    // [15, 28, 35];

const frozen = Object.freeze({ username: 'johnsmith' });
const sealed = Object.seal({ username: 'johnsmith' });

frozen.name = 'John Smith';  // frozen = { username: 'johnsmith' }
sealed.name = 'John Smith';  // sealed = { username: 'johnsmith' }

delete frozen.username;      // frozen = { username: 'johnsmith' }
delete sealed.username;      // sealed = { username: 'johnsmith' }

frozen.username = 'jsmith';  // frozen = { username: 'johnsmith' }
sealed.username = 'jsmith';  // sealed = { username: 'jsmith' }

const myObj = {
  a: 1,
  b: 'hello',
  c: [0, 1, 2],
  d: { e: 1, f: 2 }
};
Object.freeze(myObj);

myObj.a = 10;
myObj.b = 'hi';
myObj.c[1] = 4;
myObj.d.e = 0;
/*
myObj = {
  a: 1,
  b: 'hello',
  c: [0, 4, 2],
  d: { e: 0, f: 2 }
}
*/

Context

From 30-seconds-of-code: deep-freeze-object

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